Massachusetts Institute of Technology
Laser Safety Training
William B. McCarthy, Ph.D.
Radiation Protection Office
Laser Safety at MIT
• Massachusetts State Regulations
• ANSI Standard for Laser Use
• MIT Radiation Protection Office
• MIT Laser Safety Policies
• Laser Safety Officer
• Laser Safety Liaison for each department
MIT Radiation Protection Office
and the Laser Safety Officer
• Register laser systems with the State
• Register laser workers
• Instruct as to laser hazards and safe use of
lasers
• Perform hazard analysis for all Class 3b
and 4 lasers
• Provide safety related information to laser
users such as proper eyewear
• Coordinate eye exams for laser users
Laser Basics
• Light
• Amplification by
• Stimulated
• Emission of
• Radiation
Laser Light vs White Light
Laser light is:
• Monochromatic
• Directional
• Coherent
These three properties
of laser light make it
different than normal
light.
Electromagnetic Spectrum
Power
RF :
AM, FM, TV
MW
X-Rays
IR
UV
Visible Light
- Rays
Cosmic
Laser Light Spectrum
Photochemical
Effects
Retinal Damage
Ultraviolet
Visible
Near Infrared
Not “Eye Safe”
Heating Surface
of the Eye
Mid Infrared
Far Infrared
0.7
0.2
0.4
0.6
0.8
1.0
2.0
3.0
10.0
CO2
Er:YAG
Ho:YAG
Nd:YAG
GaAs
Ruby
HeNe
Argon
Eximer
Wavelength (microns)
How A Laser Works
FEEDBACK MECHANISM
ACTIVE MEDIUM
High
Reflectance
mirror
EXCITATION
MECHANISM
Output
coupler
mirror
Types of Lasers
Active Medium
- Gas
- Solid State
- Diode
- Dye
Time Duration
- Continuous Wave (> 0.25 sec)
- Pulsed (< 0.25 sec)
- Single Pulse (< 1Hz)
- Pulsed (Q-switched)
Laser Characteristics
• Power Output (watts)
– 10 W to MW
• Wavelength (nanometers)
– 200 nm to 10.6 m
• Pulse Duration and Delay
(nanoseconds - seconds)
– 1 psec to continuous
A 100 watt light bulb has
100,000 times the power of
a 1 mW laser, the same laser
is 10 million times brighter
Examples of Lasers
Laser Media
Ruby
Type
Wavelength (nm)
Solid Visible (694.3)
Application
Ranging, hole drilling, surgery
Neodymium - YAG
Solid
Carbon Dioxide
Argon
Gas
Gas
Helium Neon
Gas
Metal processing, welding, eye
surgery, military weapons and ranging
High power metal treatments, surgery
Eye surgery, cell counters, diagnostics,
photoetching, light shows
UPC checkout systems, alignment,
construction, video discs
Communications, infrared beacons in
array format, laser printers, CD players
Light shows holography, diagnostics
Gallium Arsenide
Krypton
Rhodamine
Near IR, Visible, and
UV- A
Far IR (10600)
Visible (488 – 514)
Visible and UV-A
(351, 363, 632.8)
Diode Near IR
(820, 1310, 1550)
Gas
Visible
(676,647,568,528,476)
Dye
Visible
Spectroscopy, IC circuit etching,
(Tunable 570 – 650)
photochemical reactions
Laser Safety Terminology
• MPE - Maximum Permissible Exposure - “The
level of laser radiation to which a person may be
exposed without hazardous effect or adverse
biological changes in the eye or skin.”
• NHZ - Nominal Hazard Zone - “The space within
which the irradiance or radiant exposure exceeds
the appropriate MPE.”
More Laser Terminology
• O.D. - Optical Density - Approximately the order
of magnitude of transmittance ( ). More
accurately the OD is equal to log10 (1/ ).
– Eyewear is chosen with an OD that will reduce the eye
exposure to the MPE or below.
• Irradiance - Watt/cm2
• Radiant exposure - Joule/cm2
Laser Hazards
laser safety is no joke
Laser Hazards
• Primary Beam Hazards
Fire
• Scattered Beam Hazards
Chemical
• Non-Beam Hazards
Air Contaminants
Electrical
Primary Beam
Scattered Beam
Specular vs Diffuse Reflections
• Specular - reflection from a
“shiny” object. This can be
as hazardous as the primary
beam.
• Diffuse - reflection from a
rough object. The
“roughness” depends on the
wavelength of the light.
Laser Classifications
Class 1
Class 2
Class 3a
Class 3b
Class 4
- No Biological effect
- Eye hazard for chronic viewing only
- Eye hazard for chronic viewing or use
of collecting optics
- Eye and Skin hazard for direct beam
exposure
- Eye and Skin hazard for direct beam
and scattered radiation, also a fire
hazard
• More detail on laser classification later
Skin - Biological Effects
• Thermal (all wavelengths)
– Skin burns
• surface burn from CO2 (typical
thermal burn)
• deep burn from YAG (very painful)
• Photochemical (< 550 nm)
– “tanning”
– skin cancer
– photosensitive reactions
(some are medication related)
• Shockwave (Acoustic)
Degree of Bio-effect depends on:
• Wavelength of the beam
• Irradiance or radiant exposure on the tissue
• Duration of exposure and pulse repetition
characteristics
• Extent of vascular flow to the area
• Size of the area irradiated
Tissue absorption - wavelength dependence
•
Skin is fairly transparent to red light and near infrared. Here one can see why the
CO2 laser causes a surface burn and a YAG causes a deeper burn.
Eye - Biological Effects
• Corneal Damage
– photokeratitis - welder’s flash
– corneal burn
• Retinal damage
retina
light rays focus on
the retinal back
surface of the eye
– blindness fovea/macula
– retinal detachment
– retinal burn
macula
fovea
lens
optic nerve
cornea
• Lens
– cataracts
blood supply
A detailed picture of the human eye
Ocular transmission and retinal absorption
The highest retinal absorption is in the blue region. Blue
light poses a relatively greater hazard to the retina than
other visible wavelengths. Ultraviolet and mid to far
infrared light is absorbed by the cornea. Near infrared is
absorbed by the lens and can produce cataracts.
Absorption of laser light by the eye
Visible and near infrared (0.4 - 1.4 um)
wavelengths are focused by the cornea
and lens and absorbed by the retina.
Mid and far infrared (1.4 - 1000 um)
wavelengths and UV-B and C (0.18 0.315 um) are absorbed by water on
the surface of the eye.
UV-A (0.315 - 0.390 um)
wavelengths are absorbed in the
cornea and lens structure.
Laser Eye Exam
• Eye exams are required of all personnel directly involved
with the use of Class 3b and 4 lasers. The eye exam is done
by the MIT Medical Department.
•
•
•
•
Baseline
Incident
Periodic
Termination
A Multiple Choice Question
How Do You Avoid Laser Eye Injuries?
a) Always keep your eyes closed when
working with lasers
or
b) Wear the proper eyewear for your
laser system
Laser Eyewear
• Laser eyewear should be readily available and
worn whenever a hazardous condition exists.
• RPO recommends and approves the appropriate
protective eyewear.
• Eyewear should let as much visible light through
while still blocking the laser light.
• Special Alignment eyewear should be used during
alignments.
• Also, eyewear should fit comfortably.
Laser Classification
• Class 1
– no biological effect
– less than 0.4 uW visible light
– if higher class laser is fully enclosed and interlocked it can be
classified as a Class 1 laser.
• Class 2
–
–
–
–
–
visible light only
less than 1mW
hazard for chronic viewing (>0.25 sec)
aversion response adequate protection
Class 2a supermarket scanners
Laser classification cont.
• Class 3a
– visible light only (ANSI makes some exception for invisible)
– 1 to 5 mW
– same as Class 2 except it is also a hazard with collecting optics
• Class 3b
– visible and invisible light
– 5 to 500 mW of continuous wave power
– eye and skin hazard with direct beam
• Class 4
– visible and invisible light
– greater than 500 mW
– eye and skin hazard for exposure to both direct beam and scattered
radiation
– fire hazard
Laser Warning Signs
• Class 2 and 3a lasers
Laser Radiation
- Do Not Stare into Beam
2.5 mW - HeNe - 543 nm
Class 3a laser
• Class 3b and 4 lasers
Invisible laser beam - Avoid Eye
or Skin Exposure to Direct or
Scattered Radiation
750 mW - Nd:YAG - 1064 nm
Class 4 laser
MIT Radiation Protection Office 3-2180
Laser Safety Controls
• Engineering Controls
• Administrative Controls
• Procedural Controls
• Use appropriate controls
Engineering Controls
• Operation
– remote firing
– key switch
• Laser beam
–
–
–
–
beam stop or attenuation
beam shutters
controlled beam path
protective housing
• Entry/access to laser
–
–
–
–
door interlocks
filtered windows
laser Controlled Area
warning lights
Administrative Controls
• Standard operating procedures
– startup - shutdown - specific operations - emergencies
• Administrative procedures
– limitations on use of laser according to classification
• training: Class 3b and 4 lasers must be operated by
experienced and trained operators. Spectators are subject to
entry requirements
• operating manuals
• eyewear labeling
• Laser warning signs
Using Appropriate Controls
• Class 1 - No warning signs are required
• Class 2 - Caution signs/labels are required
• Class 3a - Warning (Caution or Danger)
signs/labels are required, eyewear required if laser
beam can not be controlled
Appropriate controls cont.
• Class 3b - Danger signs/labels, beam path control,
and protective eyewear required, plus the
following are recommended:
–
–
–
–
–
–
–
protective housing
key switch
area interlocks
beam stops/attenuators
activation warning system
Class 3b laser controlled area
establish Nominal Hazard Zone (NHZ)
Appropriate controls cont.
• Class 4 - Those items recommended for Class 3b are
required for Class 4 lasers. The following are
recommended:
–
–
–
–
–
–
–
supervised by an individual knowledgeable in laser safety
require approved entry for all non-involved personnel
use diffusely reflecting materials near the beam
light tight room
remote firing or remote viewing
beam stops made of fire resistant materials
CPR training (strongly recommended)
Safe Work Practices
•
Never intentionally look directly into a laser.
•
Do not stare at the light from a laser. Allow yourself to blink if the
light is too bright.
•
Never direct the beam toward other people.
•
Remove all unnecessary reflective objects from the area near the
beam path. This may include items of jewelry or tools.
•
Never allow a laser beam to escape from its designated area of use.
•
Do not enter a designated Class 3b or 4 (posted with a Danger sign)
laser area without the proper eyewear.
•
Position the laser so that it is well above or below eye level (both
standing and sitting).
•
When not in use a laser should be stored to prevent unauthorized use
by untrained individuals.
Laser Accidents
• Electrocution at MIT
• “Accident Victim's View” page 19 of the MIT Laser Safety
Program manual.
• Over 1/2 of the reported incidents have been with graduate
students.
• Visiting Professor removes his eyewear so he can see better.
Retinal burn, permanent damage.
• Bystander not wearing eyewear during an alignment
procedure. Immediate retinal burn, lesion at edge of macula
• Professor measuring output, reflected beam into his eye.
Permanent damage to macula (blind spot). No eyewear.
Laser Warning Sign
Laser Radiation - Avoid Eye or
Skin Exposure to Direct or
Scattered Radiation
Type:________________________
Power:_______________________
Wavelength:___________________
Pulse:________________________
MIT Radiation Protection Office 3-2180
Class 4 laser